martha moore, p.e. ghyabi & associates october 10, 2014 roundabouts 101
TRANSCRIPT
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Martha Moore, P.E.Ghyabi & AssociatesOctober 10, 2014
Roundabouts 101
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What is a Roundabout?• Yield control on entry
▫ Priority to circulating vehicles
• Slow, consistent speeds• Landscaping• Pedestrian access & crossing• No parking• Direction of circulation
▫ Channelization
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What is not a modern roundabout?
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Modern roundabouts vs. traffic circles
• Yield on entry – first rule (UK 1966)• Counter-clockwise circulation• Lower speeds necessary - deflection
▫ Splitter islands▫ Central island
• Pedestrian movements restricted to crossing legs
• No parking
First Roundabout in USA – Summerlin, NV early ‘90s
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Types of Roundabouts and Typical Volumes
Type Typical Diameter Typical Entering Volumes
Mini-roundabout 45-90’ 15,000 ADT
Single lane 90-180’ 34,000
Multi-lane 150-300’ 45,000-70,000
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Roundabouts vs. 4 Way Intersection
• The “Mythbusters” TV show did a test comparing a roundabout to a 4 way stop intersection.
• During two 15-minute tests, 385 vehicles passed through the 4 way intersection.
• During the same time period, 460 vehicles passed through the roundabout intersection.
• In this test, the roundabout improved traffic flow by 20%.
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Roundabouts are being utilized under a wide variety of conditions
• Freeway interchanges• High speed rural• High volume conditions• High pedestrians• High truck volumes• Awkward geometry• Near schools• Gateways into lower speed
facilities• Light rail corridors
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Florida Roundabouts – About 280 in Florida
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Urban Roundabout ExamplesMulti-lane roundabout in
Clearwater, FLSingle lane roundabout in
Jacksonville, FL
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Key roundabout advantages – Better use of intersection space and time•Safety•Delay•Emissions•Fuel Savings•Aesthetics•Flexible to low volumes/high volumes
(doesn’t require timing plans)•Low maintenance
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Safety Advantages
• Slower speeds• Simplified decision making• Over 90% reduction in
fatalities• 76% reduction in injuries• 35% reduction in total
crashes
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Comparison of Vehicle Conflict Points
75% fewer vehicle conflict points
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Comparison of Pedestrian Conflict Points
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Safety – Increase Pedestrian Safety•Shorter crossing distance•Pedestrian only looks one direction•Drivers at signals watch the light•Drivers at typical intersection tend to look
left and turn right•Pedestrian crossing separated from
intersection – allows the driver to pay more attention to the pedestrian
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Lower speeds due to Roundabouts lead to less pedestrian deaths
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Pedestrian Safety Issues
•Disadvantage is that vehicles aren’t completely stopping
•Possible issues with visually impaired pedestrians
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Advantages – Reduced Delay
•Delay is the difference between▫Travel time with no intersection control▫Travel time with intersection control
Includes slow down, stop, and speed up time •Always better than AWSC•Usually better than TWSC if minor road
volumes are close to major road volumes 50/50, 60/40, 70/30
•Usually better than signal
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Delay Reduction – SignalizedLower Delay = Lower Emissions
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Roundabouts Save fuel & Reduce Emissions•At least two studies have noted that
roundabouts can reduce fuel consumption by about 30%.
•In one study, replacing a signal with a roundabout reduced CO emissions by 29% and Nitrous Oxide emissions by 21%.
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Where are Roundabouts Appropriate?
•Entering volumes less than 8,000 vph•Roughly same functional classification•Entry volumes balanced = higher capacity
▫The upper capacity limits of a roundabout will be higher with balanced flows.
•All levels of pedestrian volumes.
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Locations Not Recommended for Roundabouts
•Corridors with a well operating coordinated signal system
•Where satisfactory geometric designs can’t be provided ▫Deflection▫Inscribed circle diameter▫Roadway width▫Approach grades
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Where Roundabouts May Excel• Locations with unusual geometry or number
of legs• Rural intersections with high crashes or high
delay• Intersections with high amounts of left turns
on one or more legs• Interchange locations with right of way,
bridge width, or bridge crest restrictions• Near schools or playgrounds• Through small towns or commercial corridors
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Design for Appropriate Vehicle – Critical at Planning Stage
Poor Design Good Design
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Public Attitude Toward Roundabouts Before and After Construction
Attitude Before Construction
After Construction
Very Negative 23% 0%
Negative 45% 0%
Neutral 18% 27%
Positive 14% 41%
Very Positive 0% 32%
Source: NCHRP Synthesis 264
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Roundabouts and the Visually Impaired• According to the Federal Highway Administration,
people who are visually impaired may experience difficulty using roundabout crosswalks.
• This problem consists of three parts:▫ Motorists do not yield to pedestrians where the crossing
is not signal controlled.▫ At roundabouts, noise from circulating traffic may make
aural detection of gaps difficult.▫ Gaps large enough to be aurally detected may be
infrequent• The American with Disabilities Act requires that all
new and modified intersections, including roundabouts, be accessible to and usable by people with disabilities.
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Questions?